Method of purifying lithium hexafluorophosphate

This application is a continuation under Title 35, United States Code §119(e) of any U.S. application Ser. No. 09/600,231 filed Sep. 5, 2000, U.S. Pat. No. 6,514,474 in the United States Patent Office.

1. Field of the Invention

The present invention concerns a method of purifying lithium hexafluorosphate that allows purifying lithium hexafluorophosphate, useful as, for instance, lithium secondary cell electrolyte, organic synthesis medium or the like, to an extremely high purity.

2. Description of the Related Art

Lithium hexafluorophosphate is usually synthesized in an insoluble medium such as hydrofluoric anhydride or organic medium; however it reacts with a trace of water existing in the medium, and generates oxyfluoride and lithium fluoride etc., then they are mixed into the product and contaminates the same. Besides, lithium hexafluorophosphate decomposes and equilibrates partially as shown in the following expression, when it is dissolved in hydrofluoric anhydride or organic medium.

LiPF 6 ←→LiF+PF 5

This LiF is mixed into the product and becomes an impurity.

As lithium hexafluorophosphate has an extremely high moisture absorption ability, even a trace of humidity in the atmosphere provokes hydrolysis to generate the fluoric acid portion. In addition, lithium fluoride or lithium oxyfluorophosphate (LiPO x F y ) is generated to cause an impurity. If lithium hexafluorophosphate containing such impurities is used as lithium cell the electrolyte, fluoric acid erodes into electrode material, and lithium fluoride becomes a residue as it is insoluble in organic medium and requires filtration and removal, while lithium oxyfluorophosphate accelerates the decomposition of the organic medium, and possibly increases inner voltage or lowers cell capacity.

Conventionally, these impurities can be removed, by a method wherein lithium hexafluorophosphate is dissolved in an organic medium and impurities are removed by an ion exchange resin (Japanese Patent Laid-Open No. SHOU 59-87774) or a method for neutralizing in an organic medium (Japanese Patent Laid-Open No. SHOU 59-81870); however, these methods increase the production process and costs.

Moreover, a method for reacting and removing oxyfluorophosphate with fluoric gas (Japanese Patent Publication No. HEI 4-16406, Japanese Patent Publication No. HEI 4-16407) or a method for making hexafluorophosphate which contacts with a gas containing pentafluorophosphate (Japanese Patent Laid-Open No. HEI 6-29850) or the like involves danger in gas treatment and require special knowledge. Moreover, fluoric gas or phosphor pentafluoride to be used are required to be refined to a high purity, increasing cost and considerably affecting the product price.

Therefore, none of the conventional methods are satisfactory.

It is an object of the present invention to provide a method of purifying lithium hexafluorosphate that allows to obtain high purify lithium hexafluorophosphate, by converting hydrogen fluoride, oxyfluoride, lithium fluoride or other impurities, generated in the course of production, handling or conservation of lithium hexafluorophosphate and mixed with the product, into lithium hexafluorophosphate.

The method of purifying lithium hexafluorosphate of the present invention si characterized by that the purification is performed by introducing phosphoric chloride and hydrogen fluoride of a quantity necessary for producing phosphor pentafluoride of a quantity more than necessary for converting crude lithium hexafluorophosphate and oxyfluoride in the raw material into lithium hexafluorophosphate, in the reaction system with brut or dry lithium hexafluorophosphate and reaction.

Effect

In the present invention, impurities such as hydrogen fluoride, oxyfluoride, lithium fluoride contained in lithium hexafluorophosphate are converted into lithium hexafluorophosphate by adding phosphoric chloride for the purification. At this moment, phosphoric chloride and hydrogen fluoride of equal or superior to the equivalent amount are introduced for reaction.

At this moment, phosphoric chloride reacts with hydrogen fluoride to generate phosphor pentafluoride:

PCl 6 +5HF→PF 5 +5HCl  (1)

The generated phosphor pentafluoride reacts respectively with oxyfluoride and lithium fluoride as follows to be converted into lithium hexafluorophosphate and purified.

LiPO x F y +PF 5 →LiPF 6 +PO x F y−1   (2)

LiF+PF 5 →LiPF 6   (3)

The reaction shown by formula (1) develops quantitatively at room temperature and normal pressures. By-product hydrochloric gas is not involved in the reaction but discharged from the system without depositing to or remaining in the product because of high vapor pressure.

The ration of phosphoric chloride to hydrogen fluoride is desirable that hydrogen fluoride by equal or superior by chemical equivalent amount; however, even if phosphoric chloride is in excess and remains non-reacted, it can be removed by heating at a temperature equal or higher than 100° C., because its sublimation temperature is 100° C.

The reaction of phosphor pentafluoride and oxyfluoride shown in the formula (2) and the reaction of phosphor pentafluoride and lithium fluoride shown in the formula (3) depend on the temperature. The reaction develops easily if the temperature is high. However, as the temperature attains 250° C. or more, the counter reaction as shown below will occur, increasing the content of lithium fluoride as an impurity. FIG. 1 shows an example of differential thermal analysis of lithium hexafluorophosphate.

LiF+PF5←→LiPF6>250° C.  (4)

Though the reaction of phosphor pentafluoride with oxyfluoride, lithium fluoride proceeds sufficiently around the normal pressure, it is desirable that the reaction develops under pressure. If the reaction system is put under the negative pressure, not only the reaction of phosphor pentafluoride with oxyfluoride, lithium fluoride proceeds hardly, but, on the contrary, lithium hexafluorophosphate decomposed, increasing impurities. This is shown, in FIG. 2 , by the relationship between free acid and insoluble residue by the inner pressure of the reaction system. Additionally, the time necessary for this generation reaction depends on conditions such as temperature or pressure, and the content of hydrogen fluoride, oxyfluoride, and lithium fluoride in lithium hexafluorophosphate used for the generation, however several hours to a whole day and night are sufficient. When the purification reaction is completed the remaining hydrogen fluoride, phosphor pentafluoride, hydrochloride, and non-reacted phosphoric chloride gas are purged completely, through dry nitrogen gas.

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graphic showing the results of differential thermal analysis of lithium hexafluorophosphate; and

FIG. 2 is a graphic showing the relationship between inner pressure of the reaction system, free acid, and insoluble residue.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.

Best Embodiments of the Invention

(Embodiment 1)

500 g of brut lithium hexafluorophosphate containing 4% of hydrogen fluoride, 50 ppm of oxyfluoride (LiPOF 4 ) and 0.13% of lithium fluoride is taken in a PFA vessel of 1 L, 5.5 g of phosphoric chloride is added thereto and mixed well, then the vessel is sealed with nitrogen gas flow, heated a whole night at 760 Torr, 105° C. for reaction. Remaining gas is purged by nitrogen gas to obtain purified lithium hexafluorophosphate. Impurities thereof were quantified to determine: hydrogen fluoride 30 ppm, oxyfluoride 10 ppm or less and lithium fluoride 0.04%. Note that the stoichiometric amount of phosphoric chloride required for oxyfluoride and lithium fluoride being 5.3 g; it is added here by 1.0 time.

COMPARATIVE EXAMPLE 1

500 g of brut lithium hexafluorophosphate containing 4% of hydrogen fluoride, 50 ppm of oxyfluoride (LiPOF 4 ) and 0.13% of lithium fluoride is taken in a PFA vessel of 1 L, 2.7 g of phosphoric chloride, about half of required amount for converting impurities into lithium hexafluorophosphate is added thereto and mixed well, then it is operated as in (Example 1) thereafter. Impurities in the obtained product thereof were quantified to determine: hydrogen fluoride 50 ppm, oxyfluoride 30 ppm and lithium fluoride 0.08%, showing that impurities have been converted into lithium hexafluorophosphate rather insufficiently. Note that phosphoric chloride was added by half of the stoichiometric amount of Embodiment 1.

(Embodiment 2)

500 g of brut lithium hexafluorophosphate containing 150 ppm of hydrogen fluoride and 4.0 g of lithium fluoride is taken in a PFA vessel of 1 L, 4.0 g of phosphoric chloride is added there to and mixed well. Further, 2.0 g of liquid hydrogen fluoride corresponding to phosphoric chloride is added. The vessel is sealed with nitrogen gas flow, heated a whole night at 760 Torr, 105° C. for reaction. Remaining gas is purged by nitrogen gas to obtain purified lithium hexafluorophosphate. Impurities thereof were quantified to determine: hydrogen fluoride 40 ppm and lithium fluoride 0.02%. Note that the stoichiometric amount of hydrogen fluoride for 4.09 of phosphoric chloride is 1.9 g.

COMPARATIVE EXAMPLE 2

500 g of brut lithium hexafluorophosphate containing 150 ppm of hydrogen fluoride and 0.10% of lithium fluoride is taken in a PFA vessel of 1 L, 4.0 g of phosphoric chloride is added thereto and mixed well. Further, phosphoric chloride and 0.9 g of liquid hydrogen fluoride, about half of the equivalent amount, are added thereto, and then it is operated as in (Example 2) thereafter. Impurities in the obtained product thereof were quantified to determine: hydrogen fluoride 20 ppm and lithium fluoride 0.05%, showing that lithium fluoride was converted into lithium hexafluorophosphate insufficiently. Note that hydrogen fluoride was added by half of the stoichiometric amount of the Example 2.

(Embodiment 3)

1 kg of brut lithium hexafluorophosphate containing 0.5% of hydrogen fluoride and 0.11% of lithium fluoride is taken in a 3 L vessel made of SUS316, 9.0 g of phosphoric chloride is added thereto and mixed well. It is placed in a humidifier maintained at 200° C. and made to react for 6 hours. At this moment, as the vessel inner pressure has increased, it is adjusted to more or less 1500 Torr. After completion of the reaction, the inner pressure is vented, and further gas remained inside is purged by nitrogen gas to obtain purified lithium hexafluorophosphate. Impurities thereof were analyzed to determine: hydrogen fluoride 25 ppm and lithium fluoride 0.02%. Note that, in this Example, a chemical equivalent amount of phosphoric chloride necessary for lithium fluoride is added.

COMPARATIVE EXAMPLE 3

1 kg of brut lithium hexafluorophosphate containing 0.11% of hydrogen fluoride and 0.11% of lithium fluoride is taken in a 3 L vessel made of SUS316, 9.0 g of phosphoric chloride is added thereto and mixed well. It is placed in a humidifier maintained at 270 to 280° C. and made to react for 6 hours. At this moment, as the vessel inner pressure has increased, it is adjusted to more or less 1500 Torr. Thereafter, it is operated as in (Embodiment 3). Impurities in the obtained lithium hexafluorophosphate were analyzed to determine: hydrogen fluoride 20 ppm and lithium fluoride 0.2%. Lithium hexafluorophosphate has decomposed as the temperature was too high. Note that phosphoric chloride is added as much as Embodiment 3.

(Embodiment 4)

1 kg of brut lithium hexafluorophosphate containing 0.5% of hydrogen fluoride and 0.11% of lithium fluoride is taken in a 3 L vessel made of SUS316, 9.0 g of phosphoric chloride is added thereto and mixed well. It is heated to 105° C. and made to react for 10 hours. At this moment the vessel inner pressure has attained 2500 Torr. After completion of the reaction, the inner pressure is vented, and further and further gas remained inside is purged by nitrogen gas to obtain purified lithium hexafluorophosphate. Impurities thereof were analyzed to determine: hydrogen fluoride 20 ppm and lithium fluoride 0.01%. Note that the stoichiometric amount of phosphoric chloride of Example 3 is added.

COMPARATIVE EXAMPLE 4

1 kg of brut lithium hexafluorophosphate containing 0.5% of hydrogen fluoride and 0.11% of lithium fluoride is taken in a 3 L vessel made of SUS316, 9.0 g of phosphoric chloride is added thereto and mixed well. It is heated to 105° C. and made to react for 10 hours. At this moment, the vessel inner pressure has been lowered to 400 Torr by a vacuum pump. After completion of the reaction, nitrogen gas is blown inside. Impurities in the obtained lithium hexafluorophosphate were analyzed to determine: hydrogen fluoride 20 ppm and lithium fluoride 0.26%. Lithium hexafluorophosphate has decomposed by decompression and heating. Note that the stoichiometric amount of phosphoric chloride of Example 3 is added.

The present invention allows to easily purify lithium hexafluorophosphate, useful as lithium secondary cell electrolyte or organic synthesis medium. It is characterized that by harmful impurities such as oxyfluoride and lithium fluoride or the like are made to react in the presence of phosphoric chloride and hydrofluoric acid equal or superior to the equivalent amount, and oxyfluoride and lithium fluoride are converted into lithium hexafluorophosphate by means of phosphor pentafluoride. The use of phosphoric chloride is inexpensive, and the handling is easy as phosphoric chloride is solid. On the contrary, a method to refine lithium hexafluorophosphate to a high purity with the gas containing phosphor pentafluoride involves danger as gas is handled directly, and requires specialized knowledge. Moreover, the production of high purity phosphor pentafluoride required for the purification is expensive.

According to the present invention, a great effect can be expected for producing inexpensively and in quantity high purity lithium hexafluorophosphate by using phosphoric chloride in the course of purification.

While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.